As ophthalmic surgeries, there is a surgery to be performed by positioning a tip end of a cannula which is an surgical instrument to a blood clot of a blood vessel having a diameter of 0.1 mm to 0.2 mm existing in a retina of an eye fundus and puncturing the blood vessel or by positioning a tip end of a surgical instrument such as forceps, scissors, and tweezers to a fine affected part of a fundus (retina) and treating the affected part. Usually, an operator inserts the tip end of the surgical instrument in the eye through a wound opening formed in a sclera of an eyeball of a patient, positions the tip end of the surgical instrument to the affected part while observing the affected part through a microscope or the like, and treats the affected part with the surgical instrument. It is therefore preferable to make an accurate and easy positioning operation to position the tip end of the surgical instrument to the fine affected part.